Trivalent actinide and lanthanide separations by tetradentate nitrogen ligands: A quantum chemistry study

Although a variety of tetradentate ligands, 6,6′-bis(5,6-dialkyl-1,2, 4-triazin-3-yl)-2,2′-bipyridines (BTBPs), have been proved as effective ligands for selective extraction of Am(III) over Eu(III) experimentally, the origin of their selectivity is still an open question. To elucidate this question, the geometric and electronic structures of the actinide and lanthanide complexes with the BTBPs have been investigated systematically by using relativistic quantum chemistry calculations. We show herein that in 1:1 (metal:ligand) type complexes substitution of electron-donating groups to the BTBP molecule can enhance its coordination ability and thus the energetic stability of the formed Am(III) and Eu(III) complexes in the gas phase. According to our results, Eu(III) can coordinate to the BTBPs with higher stability in energy than Am(III), no matter whether there are nitrate ions in the inner-sphere complexes. The presence of nitrate ions leads to formation of the probable Am(III) and Eu(III) complexes, M(NO₃)₃(H ₂O)_n (M = Am, Eu), in nitric acid solutions. It has been found that the changes of Gibbs free energy play an important role for Am(III)/Eu(III) separation. In fact, the weaker complexing ability of Am(III) with nitrate ions and water molecules makes the decomposition of Am(NO ₃)₃(H₂O)₄ more favorable in energy, which may thus increase the possibility of formation of Am(BTBPs)(NO ₃)₃. Our work may shed light on the design of novel extractants for Am(III)/Eu(III) separation.